In many process applications the dew point of air in a particular environment must be controlled below a predetermined level. For example, in pneumatically operated fluidic systems, if the dew point of the pneumatic air is reached or exceeded, the condensed water can adversely affect associated elements. The same is true in applications utilizing compressed air.
In state-of-the-art dehydration membrane technology, such as that set forth in U.S. Pat. No. 6,083,297, to Valus et al., also assigned to the assignee of the present invention, water vapor from e.g., a compressed air supply passes through the hollow fibers of the membrane. At the same time, a small portion of the dry air product is redirected along the length of the fibers to sweep out the water vapor which has permeated the membrane. The moisture-laden sweep gas is then vented to atmosphere, and clean, dry air is supplied to the compressed air distribution system. The structure in which the present invention finds utility is set forth in Parker-Hannifin Corporation Bulletin FNS-3000/02-A, and in Parker-Hannifin Corporation Bulletin TI-3100A, both of which pertain to the Balston® SMART Dryer™ 3000 Series Membrane Air Dryers.
Since the pressure of the water vapor-containing gas is variable, and because there is always some pressure drop through the lumens of the membrane fibers, it has been difficult to design a module that permits a predetermined portion of the product gas to be returned as the sweep gas at a relatively constant rate. Prior art systems that have attempted to regulate sweep air flow include, among others, U.S. Pat. No. 5,605,564 to Collins, which conserves sweep air by controlling the upstream pressure on the sweep orifice through the use of a pressure regulator. With this process, the sweep is constant irrespective of the actual product flow. The system of the present invention takes into account the actual process flow and sweep flows accordingly.
In U.S. Pat. No. 5,160,514 to Newbold et al., sweep air is conserved by modulating an integral sweep valve within a membrane module based upon the pressure differential that exists between inlet and outlet gas caused by fluid dynamic forces, i.e. friction and turbulence etc., as the gas to be dried flows down the bore of the fiber. Although this pressure drop is related to the gas flow rate, it is also related to the bore diameter of the fibers. Considering that variation in fiber diameter and process tolerances exist for fiber ID, it would appear to produce more variability in sweep flow adjustment as opposed to systems which, as in the present invention, directly measure product flow.
In U.S. Pat. No. 6,070,339 to Cunkelman, the sweep air flow through a pneumatic controlled purge valve is reduced, however the sweep control is limited to a full 100% sweep under a fully loaded compressor and to zero sweep under a fully unloaded compressor condition. Therefore, it does not adjust sweep according to flow rate, as is the case in the present invention.
U.S. Pat. No. 3,735,558 to Skarstrom et al. pertains to a fixed sweep dryer wherein the sweep flow is controlled by a fixed orifice or valve which is adjusted by the manufacturer for a predetermined level of dryness. In contrast thereto, the present invention pertains to a variable sweep dryer.
Similarly, U.S. Pat. No. 6,296,683 B1 to Koch, pertaining to a dryer for compressed air, utilizes a membrane dryer with a manual valve, such as a needle valve, to set the sweep flow. The manufacturer of this device, or the user thereof, can adjust the amount of sweep gas “manually” to obtain differing levels of drying performance. Once the valve is set, it acts like a fixed orifice and the volume of sweep gas is not changed unless the valve is readjusted and is thus, in effect and contrast to the present invention, a fixed sweep system.